Ultra short wave oscillator



NOV. 2, 1948. J, H, FREMUN 2,452,559

ULTRA SHORT WAVE OSCILLATOR Filed Jan. 20. 1945 r O H r- Inventor John ear/e7 firm/i7 Patented Nov. 2, 194% ill EHCE

ULTRA SHORT WAVE OSCILLATOR John Heaven Fremlin, London, England, assignor to Standard Telephones and Cables Limited, London, England, a British company Application January 20, 1945, Serial No. 573,772 lln Great Britain November 19, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires November 19, 1963 7 Claims.

plied to the resonator or resonators. While this effect may in some cases be utilized to provide a tuning adjustment for the oscillator, most usually it is an undesirable effect, and it is necessary to adopt some means for eliminating or reducing it. There are two general methods of attack: first, the device may be designed in such a manner that the frequency changes produced by variations in the operating voltage are made negligible; or second, means may be provided for compensating the efiect of such variations. The invention is concerned. with the second method, and its principal object is to provide a compensating circuit for an electron velocity modulation device adapted substantially to neutralize the frequency variation produced by a fluctuating source of voltage.

According to the invention, the operating voltage for the oscillator is supplied through a circuit which causes the electron beam current to vary in accordance with the variations in the operating voltage, and thereby to produce compensating variations which operate so that the effective electron accelerating voltage remains substantially constant.

The invention will be described with reference to the figure on the accompanying drawing, which shows a schematic circuit diagram of a stabilizing arrangement according to the invention.

Referring to this figure, the electron Velocity modulation oscillator taken as an example of the application of the invention com rises a coaxial line resonator 2 excited by an electron beam derived from a cathode and fired through a control grid 3 and through a slot defined by fins on to a collecting anode i. This is an arrangement generally in accordance with that described in patent specification No. 2,320,860 and may be embodied in any convenient practical form.

The anode i and resonator 2 are connected through a resistance R1 to the positive terminal 9 of a high tension source having a nominal voltage V liable to variations. The negative terminal ill of this source is connected to the cathode t. A resistance R2 is connected to shunt the terminals 9 and ill, and an adjustable tapping point 5 is connected through a negative counteracting bias source 6 of Voltage '0 to the control grid 3. The source. 5 is shown as a battery but may be any convenient arrangement for producing a negative counteracting bias. A gas discharge tube 5 is shown connected across the upper part of the resistance R2, but is optional and can be omitted. It will first be assumed that the tube 1 is omitted.

In order to maintain the generated frequency constant, it is necessary that the voltage E applied between the resonator 2 and cathode 4 should be independent of the variations of V; in other words the differential coefficient dE W should be zero.

In a device of this kind, the relation between the beam current I and the voltages e (the control grid voltage) and E is of the form in which a, b and n are constants determined by the details of the geometrical design of the device. Furthermore, it is evident from the figure that E=V-IR1 (2) and e=mV-v (3) dE W=0 when mammal =1 or by using Equations 1 and 3 this condition becomes Of these quantities, E and I are determined by the conditions of operation of the device, and b and n by its geometrical design. This leaves V, 2), mand R1 to be chosen in accordance with Equation 5 and this may be done within. certain limits independently of the required operating conditions. There is not a complete freedom of choice of even three of these owing to other conditions not considered above, for example 6 must be negative, R1 must be positive, and m a positive fraction. However, generally these quantities can be chosen to satisfy one other condition. An ex..-

ample of the choice of such an additional condition will appear from the following considerations. Arrangements are often required in which a generator needs to be automatically kept in tune with incoming signals, for example. In such a case the frequency is often varied by means of a control voltage applied to a control grid such as 3 in the figure. It is then essential that the accelcrating voltage shall not be subject to fluctuations.

The stabilizing arrangement described permits automatic frequency control to be applied directly to an oscillator operated from a fluctuating source. Referrin to the accompanying figure, the control voltage would be applied to the grid 3 in series with the source 6. If the operatingvoltage E applied to the resonator 2 is maintained constant by the choice of the conditions according to Equation 5, then the desired frequency variation can be obtained by means of the control voltages uncomplicated by the variations of V. Insuch an arrangement it is desirable that the power P supplied by the operating sources should be independent of the grid voltage e, or in other words 1 e1 dc This gives an extra condition for determining the quantities concerned in Equation 5. If it is arranged so that then the control voltage which determines the frequency will not modulate the power input, and then if the operating conditions are chosen for maximum efficiency the power output of the de- I vice will be considerably less affected over the range of automatic frequency variation than if the accelerating voltage E were directly controlled.

Now the variation of e affects the value of P through the variation of I, and therefore dP dP and will be zero when V=ZR1I. Thi means that B: should be chosen so that the power dissipated power oscillator is required. For such a case it has hitherto been necessary to use a low power oscillator supplied by an independent or individual low power source which does not fluctuate appreciably, or which can be easily stabilized, and to follow it with a power amplifyin stage which can be supplied from a high power source which need not be very constant. The stabilizin arrangement whichhas been described can be appliedidirectly to a .high power oscillating device supplied from a variable high power source, thus eliminating the power amplifier, and avoiding the necessity for attempting to stabilize the power source itself.

Referring again to the figure, the discussion has so far been on the assumption that the gas discharge tube 1 is omitted. If this be included, the effect is to produce a substantially constant voltage drop across the upper part of the resistance R2 so that the whole of the variation of V and not only the fraction m thereof appears across the lower part. The effect is to increase the effect of R1 by the factor l/m, so that condition (5) now takes the form This results in a smaller power dissipation in R1 for given values of the other components. If desired several such gas discharge tubes may be connected in series in the place of the tube 7 in order to stabilize a larger proportion of V.

It will be observed that the elimination of frequency modulation caused by the variation of the high tension source has been accomplished in the arrangements described by converting it into amplitude modulation which is generally far less objectionable. Thus in an electron velocity modulation device with the usual power supplies, variation of the high tension voltage produces a large frequency modulation and a moderate amplitude modulation, while variation of the control grid voltage produces a large amplitude modulation, and also a small indirect frequency modulation through the variation of the beam current, the amount of which depends on the regulation of the high tension supply. When the circuit of the invention is used, variation of the high tension voltage produces amplitude modulation but no frequenc modulation, while variation of the control grid voltage produces frequency modulation with Or without amplitude modulation depending on the manner in which condition (5) or (6) has been satisfied.

If the value of E chosen is that which gives maximum output power for the device, then any amplitude variation due to changes in V will be much reduced. Such a value of E generally exists in velocity modulation devices.

It will be understood that although a co-axial line resonator is employed in the device illustrated on the accompanying drawing, substantially the same arrangements could be used for devices having two separate hollow resonators, or for devices which employ external electric circuits instead of hollow resonators.

What is claimed is:

l. A wave generating arrangement includin a discharge device of the velocity modulating type comprising a cathode for producing an electron beam, a collector electrode, a cavity resonator arranged in the path of said beam between said cathode and collector electrode and provided with apertures for passage of the beam therethrough, a source of variable voltage, first means for connecting said source between said resonato and collector electrode and said cathode for accelerating the electrons in said beam, a control grid adjacent said cathode about said beam path, and second means for connecting said source to said grid for controlling the beam density, said first and second connecting means including a compensating circuit for maintaining the potential applied to said collector electrode and resonator constant.

2. An arrangement as setforth in claim 1 in which said compensating circuit comprises a resistance connected across said source and a center tap connected to said control grid.

3. An arrangement as set forth in claim 2 in which said compensating circuit further comprises an impedance connected to said collector electrode, in series with said source, said impedance being adjusted to a value similar to that of the equivalent impedance between said cathode and said collector electrode.

4. An arrangement as set forth in claim 3 in which said compensating circuit further comprises a source of voltage connected in series between said tap and said control grid for further adjusting the potential on said grid.

5. An arrangement as set forth in claim 4 in which said compensatin circuit further comprises a gas discharge device connected between said center tap and the side of said source connected through said impedance to said collector electrode.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,487,103 Knoop Mar. 18, 1924 1,541,311 Anderson June 9, 1925 1,959,010 Tellegen May 15, 1934 2,274,364 Gardiner Feb. 24, 1942 2,320,860 Fremlin June 1, 1943 

